Mix 8 ounces of baking soda, 4 ounces of dry citric acid, 4 ounces of corn starch and 4 ounces of sea salt in a mortar. With a pestle, ground the ingredients of the mortar to a fine powder. Make sure no lumps remain and the ingredients are thoroughly mixed with each other. Use a glass pestle and mortar if possible because glass is non-reactive and will not start an undesired reaction with the ingredients.
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In a glass jar, pour 0.75 tablespoon water, 2 tablespoon fragrance or essential oil such as ginger oil, 2.5 tablespoon vegetable oil and 1 or 2 drops of any food colour that you like. Mix the contents of the glass jar by shaking them up. The contents may look separated from each other no matter how well you shake the jar, but you do not need to worry about that.
Slowly pour the mixture in the glass jar in the mortar, adding small amounts and whisking while pouring. If you are too quick while adding the glass jar contents to the dry ingredients in the mortar, a reaction will start which will cause the resulting mixture to foam. So make sure to do this step slowly. When you are done, you will end up with a damp sandy mixture which will clump together upon squishing.
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Now you have very limited time to get the final mixture into moulds so act quickly. Get the mixture into moulds of your choice. The moulds may be of any shape such as round, oval, heart shaped etc. Also, you can use all sorts of things as moulds such as ice cube trays, muffin tins, candy moulds etc. In the first go, you may not be able to fill the right amount of mixture but that should not worry you because you will get a feel of the right amount of filling in may be three or four tries.
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If the mould you used involved too much details, you will have to let the mixture dry before you can remove it from the mould. Otherwise, you can remove it from the mould as soon as you finish filling it. You have successfully made bath bombs. In case you want to store the bath bombs, use an air tight glass gar.
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